Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system, such as that disclosed in U.S. Pat. No. 6,516,227, which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. As shown in FIGS. 1A, 1B, 2A, and 2B, a Spinal Cord Stimulation (SCS) system typically includes an Implantable Pulse Generator (IPG) or Radio-Frequency (RF) transmitter and receiver 100 (collectively, “IPGs”), at least one electrode lead 102 and/or 104 having a plurality of electrodes 106, and, optionally, at least one electrode lead extension 120. The electrodes 106 are arranged in a desired pattern and spacing on the lead(s) 102, 104 to create an electrode array 110. Wires 112, 114 within one or more leads(s) 102, 104 connect each electrode 106 in the array 110 with appropriate current source/sink circuitry in the IPG 100.
In an SCS application, the electrodes lead(s) 102, 104 with the electrodes 106 are typically implanted along the spinal cord 19 (FIG. 2B), and the IPG 100 generates electrical pulses that are delivered through the electrodes 106 to the nerve fibers within the spinal column. The IPG 100 body itself is normally implanted in a subcutaneous pocket, for example, in the patient's buttocks or abdomen. The electrode lead(s) 102, 104 exit the spinal column and generally attach to one or more electrode lead extensions 120 (FIG. 2), which in turn are typically tunneled around the torso of the patient to the subcutaneous pocket where the IPG 100 is implanted. Alternatively, if the distance between the lead(s) 102, 104 and the IPG 100 is short, the electrode lead(s) 102, 104 may directly connect with the IPG 100 without lead extensions 120. For examples of other SCS systems and other stimulation system, see U.S. Pat. Nos. 3,646,940 and 3,822,708, which are hereby incorporated by reference in their entireties. Of course, an IPG 100 is an active device requiring energy for operation, which may be provided by an implanted battery or an external power source.
Precise placement of the lead(s) 102, 104 relative to the target nerves is important for achieving a satisfactory physiological response, and for keeping stimulation thresholds low to conserve battery power. A conventional lead implantation procedure commonly places the leads 102, 104 parallel to the spinal cord column 19 at or near the physiological midline 91, as is shown in FIGS. 3A and 3B. More particularly, and as best shown in the cross section of FIG. 3B, the electrode leads 102, 104 are placed directly on the dura mater 51 within the epidural space 70. (Cerebro-spinal fluid 72 is between the electrode array 110 and the white matter 52 of the spinal cord 19. Dorsal root nerves 50 are shown emanating from grey matter 53). When the leads 102, 104 are placed on opposite sides of the physiological midline 91 as shown, additional flexibility is provided in the ability to recruit (i.e., stimulate) nerves in the dorsal column, and to treat symptoms manifesting on either the left or right sides of the patient's body.
In addition to precise placement of the electrode array, proper selection of the electrodes, i.e., determining which of the electrodes 106 in the array should be active in a given patient, is critical for achieving effective stimulation therapy. However, because of the uncertainties of the distances of the electrodes from the neural target, the unknown nature of the specific conductive environment in which the electrode is placed, etc., it generally cannot be known in advance and with precision which combination of active electrodes will be perceived by a patient as providing optimal therapy. As a result, patient therapy generally requires that various electrode combinations be tried and feedback received from the patient as to which of the combinations feels most effective from a qualitative standpoint.
Various electrode combinations and other stimulation parameters can be tried during initialization by programming the IPG 100 using an external wireless clinician or hand-held controller. (Details concerning such controllers can be found in U.S. Patent Publication 2007/0239228, published Oct. 11, 2007, which is assigned to the present application and which is incorporated herein by reference in its entirety). For example, and as best visualized in FIG. 3A, the IPG 100 can be programmed such that electrode E1 comprises an anode (source of current), while E2 comprises a cathode (sink of current). Or, the IPG 100 can be programmed such that electrode E1 comprises an anode, while E9 comprises a cathode. Alternatively, more than one electrode can be used in both the sourcing and sinking of current. For example, electrode E1 could comprise an anode, while both E2 and E9 can comprise cathodes. The amount of current sourced or sunk can also be programmed into the IPG 100. Thus, in the last example, electrode E1 could sink 5 mA, while electrode E2 sources 4 mA and electrode E9 sources 1 mA. The frequency of electrode stimulation pulses, as well as the pulsewidth or duration of such stimulation pulses, is also programmable. As disclosed in the incorporated '228 Publication, the time of no stimulation between pulses is preferably greater than or equal to 3 milliseconds. See '228 Publication, ¶ 65.
Ultimately, which electrodes are activated by the IPG 100, and the polarities (cathode v. anode), magnitudes (amount of current), and frequencies of those activated electrodes, are based largely on patient feedback during IPG initialization as noted earlier. Thus, the patient, perhaps assisted by a clinician, will experiment with the various electrode settings, and will report relative levels of comfort and therapeutic effectiveness to arrive at electrode settings that are best for a given patient's therapy.
In the prior art, patients and/or clinicians used a technique called “field steering” or “current steering” to try and simplify the iterative process for determining a patient's optimal electrode settings during initialization of the IPG. See, e.g., U.S. Pat. No. 6,909,917, which is incorporated herein by reference in its entirety. In current steering, the current sourced or sunk by the electrodes is gradually redistributed by the patient or clinician to different electrodes using a single stimulation timing channel. Such steering can be facilitated using some sort of user interface associated with the external controller, such as a joystick or other directional device. Simple examples of current steering are shown in FIGS. 4A, 4B, and 5. Starting first with FIG. 4A, assume that the IPG 100 has an initial condition, namely that electrode E1 has been programmed to sink 10 mA of current, while electrode E3 has been programmed to source 10 mA of current. This initial condition might be arrived at after some degree of experimentation, and might be a condition at which the patient is feeling a relatively good response, but a response which has not yet been fully optimized.
In an attempt at further optimization, current steering can commence from these initial conditions. Assume that optimization by current steering will ultimately arrive at the final condition of FIG. 4B. As shown, this final condition sinks 10 mA at electrode E2. Thus, during current steering, 10 mA of sink current is moved from E1 (the initial condition) to E2 (the final condition). To do this, electrode E1 is selected and the current sunk from that electrode is moved downward, for example, by clicking downward on the controller's joystick. As shown in FIG. 5, this moves some increment of sinking current (as illustrated, a 2 mA increment) from electrode E1 to electrode E2, such that E1 now sinks 8 mA and E2 sinks 2 mA. Another downward click moves another 2 mA, so that now E1 sinks 6 mA and E2 sinks 4 mA, etc., until the full 10 mA is moved to E2 as per the final condition.
Gradual steering of the current in increments is generally considered advisable to safeguard against abrupt changes of the stimulation field which may be uncomfortable or dangerous for the patient. Abrupt shifting of the entirety of the current from one electrode to another could have unforeseen and undesirable effects. Different nerves are affected by such a change in electrode activation, and it is not necessarily known how moving a full allotment of current would affect those nerves. If the current when applied to the new electrodes (e.g., from E1 to E2) is too low (i.e., sub-threshold), no clinical response would be noticed, even if the electrodes were ultimately suitable choices. If the current is too high (i.e., supra-threshold), the result might be painful (or dangerous) for the patient. Accordingly, incremental movement of the current is considered a good approach.
However, the illustrated current steering approach requires two different electrodes (e.g., E1 and E2) to simultaneously act as current sinks during the intermediate steering steps. This can be an implementation problem in IPG architectures that don't allow the simultaneous selection of two or more electrodes to act as the source or sink. For example, some simpler IPG architectures may provide only a single current source circuit and a single current sink circuit, which circuits can only be coupled to one electrode at a time. Because such architectures will not support simultaneous activation of two or more electrodes as sinks or sources, the current steering approach of FIG. 5 can't be used.
Other current steering approaches provide additional complexities. For example, the current steering approach illustrated in FIG. 6 is disclosed in U.S. Patent Publication 2007/0239228, which was incorporated by reference above. In this approach, steering of the current from one electrode to another occurs by establishing the steered current in a second timing channel. (Because the operation of timing channels are explained in detail in the '228 publication, they are not further explained here). Thus, and as shown, current in the transferring electrode (E1) is initially established in a first timing channel ‘A.’ As the current is incrementally steered to receiving electrode E2, that steered current forms in a second timing channel ‘B,’ such that the pulses in timing channel A and B are non-overlapping. The result after several incremental transfers of current is the final condition in which the sink current resides entirely with electrode E2 is in the second timing channel B.
This approach of the '228 publication thus requires IPG hardware and software necessary to support different timing channels. Not all IPGs will have such hardware or software, and so will be unable to benefit from the current steering technique of FIG. 6. Even in those IPGs that can support multiple timing channels, such a current steering technique is relatively complex, and is potentially limited. For example, although not shown in FIG. 6, one skilled in the art will understand that the pulses must generally be followed by either a passive or active current recovery period. Because pulses in the next timing channel cannot be executed until currently recovery of the pulses in the preceding timing channel is completed, the ability to use the '228 publication's current steering technique is not guaranteed. For example, if the stimulation pulses are of long duration or of a high frequency, there may simply not be enough time in which to interleave the pulses in the two timing channels, especially when current recovery periods are considered.
Accordingly, what is needed is an improved method for optimizing electrode activation during the set up of an implantable stimulator device, and this disclosure provides embodiments of such a solution.